Back when Diesel used to cost the same or less than gasoline, I used to think this was a great idea and was one the bandwagon of asking why there wasn't one available in a 1/2 ton. I often thought that something with the hp of a small V8 and the torque of the big V8's would be perfect. (IE: This is not the king of the hill engine, but optimized for excellent fuel mileage and still capable).
More recently, Diesel has consistently been running a fair amount over gasoline. In the short term, gasoline was running $3.15 a gallon, while diesel was consistently between $3.90 and 4.20 a gallon in my area. Right there was a 25%ish premium on the fuel price that would need to be offset with the diesel. Add on the premium price tag, and the payback will take a long while. Gas has started to go back up, but diesel is still a 50 cent a gallon premium.
Add on to that the "fun" of running a diesel in arctic type cold, plus the advances made in fuel mileage in gasoline engines, and while in theory I like the idea of a diesel 1/2 ton, I'm not sure the economics are there...